Jenkins is designed to set an automatic fail when the exit code of your script is 1. There are plenty of ways of overriding this, but most involve Jenkins CLI or a plugin. Using the returnStatus flag for shell commands https://jenkins.io/doc/pipeline/steps/workflow-durable-task-step/, Jenkins will not automatically fail but instead, return the status code.
Here's the documentation:
returnStatus (optional)
Normally, a script which exits with a nonzero status code will cause the step to fail with an exception. If this option is checked, the return value of the step will instead be the status code. You may then compare it to zero, for example.
script{
def returnVal = sh(
returnStatus: true,
script: '''
**your script here**
'''
)
if(returnVal != 0) {
currentBuild.result = 'UNSTABLE'
}
}